1. Field of the Invention
The present invention relates to a device for measuring an absolute displacement, which is applicable to a linear encoder, a lightwave interferometer for measuring a length, and the like.
2. Description of the Related Art
Devices for detecting a mechanical displacement of an object have been know in the art as a linear encoder of scale type and a lightwave interferometer, for example. These displacement detection devices include in principle an incremental type for detecting a relative displacement and an absolute type for detecting an absolute displacement. The incremental type device employs a displacement detector that generates a sine wave signal in response to the mechanical displacement, and counts the number of periods of the resultant sine wave signal to obtain a relative displacement (position). The absolute type device employs displacement detectors that generate a plurality of sine wave signals with different signal periods, detects phases of respective sine wave signals, and composes the resultant position information to obtain an absolute displacement (position).
Specifically employed in the case of the absolute type device are such displacement detectors that generate, for example, three sine wave signals with greatly different periods from each other. The sine wave signal with the largest period is a coarse resolution signal; the sine wave signal with a smaller period than the largest period is a medium resolution signal; and the sine wave signal with much smaller period is a fine resolution signal. A high precise absolute displacement can be obtained within a period of the coarse resolution signal by performing a process of interpolating the position information of the coarse resolution signal corresponding to an objective displacement with the medium resolution signal and further interpolating it with the fine resolution signal.
As described above, the absolute displacement measuring device requires the plural displacement detectors to generate the sine wave signals with greatly different periods. An absolute measurement can not be performed in principle within an additional measuring range that exceeds the originally prepared measuring range of the displacement detector. Then, there is proposed a wavelength composing method that employs displacement detectors prepared to generate two sine wave signals with close periods and composes their outputs to obtain a sine wave signal with a larger period, from which an absolute measurement can be performed within a broader measuring range that is not planned originally in the displacement detectors (for example, in JP 59-79114A).
In the wavelength composing method proposed in the art, a finite difference is calculated between a triangular wave signal with a period of P1 as shown in FIG. 12A and a triangular wave signal with a period of P2 as shown in FIG. 12B to obtain a composed signal with a period of P1xc2x7P2/(P2xe2x88x92P1) as shown in FIG. 12C. The closer the periods P1 and P2, the larger the period of the synthesized signal, P1xc2x7P2/(P2xe2x88x92P1). By using the composed signal as the coarse resolution signal and calculating the number of the period P1 or P2 and a fraction distance within the coarse resolution signal, an absolute position can be obtained within a period of the coarse resolution signal.
In the wavelength composing method proposed in the art, however, an error occurs at a transition of the period as shown in FIG. 12D when calculating the finite difference simply. Therefore, correction arithmetic is inevitably required to correct the error in order to obtain such the composed signal as shown in FIG. 12C. Instead of performing such the correction arithmetic, there is another detecting system that squares a beat signal that is generated from two modulated signals when they are mixed, and detects it through a low pass filter. This detecting system, however, has a disadvantage that requires a precise modulation.
In the conventional wavelength composing method, it is not considered to perform a multiple composing that further composes the composed period signal with another period signal. Because the above described correction arithmetic and modulation are strongly required to have a higher precision when the period of the composed wavelength becomes extremely greater than the period of the original signal. If such the multiple composing is not performed, two displacement detectors with extremely close periods are required to obtain a period signal with a larger measuring range. To prepare such the displacement detectors, in the case of electrical displacement detectors, for example, it is difficult to obtain a scale with a very small difference in graduations due to a limitation to accuracy for processing graduations of the scale. In addition, in the case of optical displacement detectors, it is difficult to realize a wide measuring range because a wavelength of a light source can not be selected freely with ease.
An object of the present invention is to provide a device for measuring an absolute displacement, which is capable of measuring an absolute displacement based on a long period signal obtained from a wavelength composition with a simple arithmetic.
A first aspect of the present invention is provided with a measuring device for absolute measurement of displacement, comprising: a first displacement detector for generating a sine wave signal S1A and a cosine wave signal S1B both with a period of P1 in accordance with a displacement L to be detected; a second displacement detector provided in parallel to the first displacement detector for generating a sine wave signal S2A and a cosine wave signal S2B both with a period of P2 different from the period of P1 in accordance with the displacement L to be detected; a wavelength composing circuit for composing the sine wave signals S1A, S2A and the cosine wave signals S1B, S2B from the first and second displacement detectors to obtain a sine wave signal, SxA=S1Axc2x7S2Bxe2x88x92S1Bxc2x7S2A, and a cosine wave signal, SxB=S1Bxc2x7S2B+S1Axc2x7S2A, both with a period of Px equal to the least common multiple between the periods P1 and P2; a phase detector for detecting a phase of a fine resolution signal comprising either of output signals obtained from the first and second displacement detectors and a phase of a coarse resolution signal comprising the output signal from the wavelength composing circuit; and a distance arithmetic circuit for computing the displacement L as an absolute position within the period Px based on phase information detected by the phase detector.
A second aspect of the present invention is provided with a measuring device for absolute measurement of displacement, comprising: a first displacement detector for generating a sine wave signal S1A and a cosine wave signal S1B both with a period of P1 in accordance with a displacement L to be detected; a second displacement detector provided in parallel to the first displacement detector for generating a sine wave signal S2A and a cosine wave signal S2B both with a period of P2(xe2x89xa0P1) in accordance with the displacement L to be detected; a third displacement detector provided in parallel to the first and second displacement detectors for generating a sine wave signal S3A and a cosine wave signal S3B both with a period of P3 (xe2x89xa0P2, P1, and |P3xe2x88x92P2|xe2x89xa0|P2xe2x88x92P1|) in accordance with the displacement L to be detected; a first wavelength composing circuit for composing the sine wave signals S1A, S2A and the cosine wave signals S1B, S2B obtained from the first and second displacement detectors to obtain a sine wave signal, SxA=S1Axc2x7S2Bxe2x88x92S1Bxc2x7S2A, and a cosine wave signal, SxB=S1Bxc2x7S2B+S1Axc2x7S2A, both with a period of Px equal to the least common multiple between the periods P1 and P2; a second wavelength composing circuit for composing the sine wave signals S2A, S3A and the cosine wave signals S2B, S3B obtained from the second and third displacement detectors to obtain a sine wave signal, SyA=S2Axc2x7S3Bxe2x88x92S2Bxc2x7S3A, and a cosine wave signal, SyB=S2Bxc2x7S3B+S2Axc2x7S3A, both with a period of Py equal to the least common multiple between the periods P2 and P3; a third wavelength composing circuit for composing the sine wave signals SxA, SyA and the cosine wave signals SxB, SyB obtained from the first and second wavelength composing circuits to obtain a sine wave signal, S1A=SxAxc2x7SyBxe2x88x92SxBxc2x7SyA, and a cosine wave signal, S1B=SxBxc2x7SyB+SxAxc2x7SyA, both with a period of PI equal to the least common multiple between the periods Px and Py; a phase detector for detecting a phase of a fine resolution signal comprising either of output signals obtained from the first through third displacement detectors, a phase of a medium resolution signal comprising an output signal from the first or second wavelength composing circuit, and a phase of a coarse resolution signal comprising the output signal from the third wavelength composing circuit; and a distance arithmetic circuit for computing the displacement L as an absolute position within the period PI based on phase information of the fine, medium and coarse resolution signals detected by the phase detectors.
According to the present invention, an absolute position can be obtained precisely within a period of a composed bi-phase sine wave signal with a long period, which is obtained through a wavelength composing with a simple arithmetic including multiplication, addition and subtraction of two bi-phase sine wave signals. In addition, according to the present invention, a coarse resolution signal with a high period precision can be obtained by performing multiple wavelength composing, in which two bi-phase sine wave signals are obtained by performing wavelength composing among three bi-phase sine wave signals and are further directed to another wavelength composing. As a result, an absolute position can be measured precisely within a wider measuring range.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.